1. Field of the Invention
This invention relates to a semiconductor package in which a semiconductor is mounted on a substrate and, more particularly, to an electronic circuit board including a semiconductor package which is high in packaging density, connecting reliability and yield.
2. Description of the Related Art
As great advances have been made in electronics technology such as computer technology in recent years, demands for high density, high function and high performance are on the rise. To this end, various attempts have hitherto been made to improve either semiconductors or substrates on which various electronics components including semiconductors are to be mounted.
Basically, a semiconductor package includes a semiconductor and can accommodate an electrical connection of the terminal of the semiconductor with the terminal of an electronic circuit substrate surface on which the semiconductor package is mounted, and has a function for temporarily or permanently sealing the semiconductor.
In an effort to make this semiconductor high in function and density, there is a trend that the diameter and pitch of connecting pads of the semiconductor should be reduced, There is another trend that the width and pitch of wiring should be reduced in an effort to make an electronic circuit substrate, on which a semiconductor package and various electronics components are to be mounted, high in density. Consequently, a trend has arisen in that the wiring board of a semiconductor package should become high in density.
However, since the substrate of this semiconductor package also has a sealing function as mentioned above, a highly airtight strong substrate, usually a ceramic substrate, has been used.
For production of this ceramic substrate, in general, wiring is formed on unfired ceramic sheets at their surfaces and through holes such as by printing and are then laminated, whereupon the resulting laminated substrate is sintered. During this sintering, the shrinkage coefficient of ceramics varies relatively widely so that a connecting pad on the ceramic substrate and a semiconductor pad to be mounted on the connecting pad will be mislocated (e.g., misaligned) with respect to each other. Also, mislocation would occur between the connecting pad on the substrate on which the semiconductor package is to be mounted, and the connecting pad on the ceramic substrate to be connected with the former connecting pad. Consequently, in the case where connection is made by soldering, incorrect or inadequate connection would occur to lower the yield. Various attempts have been made to avoid this problem; for example, the diameter and pitch of connecting pads are increased, or the size of semiconductors and substrates is reduced. These conventional measures are not suitable for production of high-density substrates.
To this end, a solution has been proposed in which a thin film circuit of polyimide is formed on a ceramic multilayer substrate and then a semiconductor is mounted on the resulting substrate (for example, Unexamined Japanese Patent Publication (KOKAI) No. SHO 62-122258). According to this prior art, since the thin film circuit, like the semiconductor, is formed by photolithography, it is possible to form a fine high-precision pattern so that the precision of a plane position of a pad on which the semiconductor is to be mounted can be increased, thus reducing the possibility of planar mislocation.
A disadvantage of the resulting package is that since the coefficient of linear expansion of polyimide is considerably larger than that of a ceramic substrate, the entire substrate would become warped. As a result, the gap between the substrate and the semiconductor varies locally. Further, on the back surface of the package, planar mislocation would not be eliminated.
With this prior method, it is impossible to prevent any displacement between a semiconductor package and a semiconductor and any displacement between the semiconductor package and an electronic circuit board on which the semiconductor package is to be mounted, so that a high-density semiconductor package and even a high-density electronic circuit board cannot be realized.